A sodium-containing quasicrystal: using gold to enhance sodium's covalency in intermetallic compounds.

Quasicrystals (QCs) exhibit crystallographically forbidden rotational symmetries and aperiodic long-range positional order. Three-dimensional quasicrystals, that is, icosahedral quasicrystals (i-QCs) belong to one of three commonly accepted subtypes: 1) Bergman; 2) Mackay; and 3) Tsai types, which are differentiated by their atom clusters that are also essential structural components of corresponding crystalline approximants. Approximants are periodic crystals with similar chemical compositions to nearby QCs. After the discovery of the first i-QC in rapidly quenched Al–Mn, many stable and metastable species have been synthesized with elements that span major portions of the periodic table (Figure 1a). None of these i-QCs nor any other QC, however, contain Na, although numerous Na-containing Bergman-type structures, such as Na16Mg36Al40Zn68, [4] Na13Cd20Pb7, [5] Na13Cd18.9Tl8.1, [6] Na26Au40.9Ge14.1, and Na26Au39.8Sn15.2, [7] have been reported. Herein, we report the discovery and characterizations of the first Na-containing i-QC, i-Na13Au12Ga15, which belongs to the Bergman type but has an extremely low valence electron-to-atom (e/a) value of 1.75 for such phases (Figure 1b). By analyzing the electronic structure of the 1/1 approximant structure, the existence and stability of this Na-containing i-QC is tightly linked to its substantial Au content, which allows the e/a value to satisfy a Hume–Rothery stabilization rule and creates novel Na–Au polar-covalent interactions. This new i-QC was discovered during systematic exploration of the Na–Au–Ga system to uncover novel polar intermetallics with complex Au–Ga frameworks. Phases that were identified in the narrow region near 32 at%Na with Au/ Ga molar ratios ranging from approximately 1:2 to 2:1 (see Figure 2) include a stuffed (Ga-centered) Bergman 1/1 phase,